Recently, demand for a radio device for mobile communications sharply increases, variety of the form of the radio device increases, and transmission and receiving of more information by one radio device is required. A radio device capable of transmitting and receiving radio waves in a plurality of frequency bands is sold, and the radio device employs a multi-band-compatible antenna device available in a plurality of frequency bands.
A portable phone is described as a radio device for mobile communications employing such an antenna device.
Portable phones are used in various areas in the world, and a different frequency band is employed in each area. In a digital portable phone, for example, the frequency band of a global system for mobile communications (GSM) is 880 to 960 MHz, that of a digital communication system (DCS) is 1710 to 1880 MHz, and that of a personal communication system (PCS) is 1850 to 1990 MHz.
As the portable phone is downsized and made compact and the number of bands increases, the number of antenna devices built and used in the portable phones is apt to increase and the demand for downsizing the antenna devices grows.
As a conventional multi-band-compatible antenna device built in the portable phone, a plate inverted-F antenna shown in FIG. 3 is described.
FIG. 3 is a perspective view of a conventional plate inverted-F antenna supporting two frequency bands, GSM and DCS. In FIG. 3, first antenna element 21 is an antenna supporting GSM, and second antenna element 22 is an antenna supporting DCS. These plate inverted-F antennas are mounted on wiring board 23 of a portable phone (not shown). Second antenna element 22 may support PCS.
In the plate inverted-F antenna, first antenna element 21 supporting low frequencies and second antenna element 22 supporting high frequencies face wiring board 23 in parallel, and are disposed on the same plane side by side. Desired element width, length, and inter-element distance are set for first antenna element 21 and second antenna element 22 so as to provide respective required radiant efficiencies.
One end of first antenna element 21 and one end of second antenna element 22 are integrally interconnected, and are connected to the ground (GND) terminal (not shown) and feeding point (not shown) of wiring board 23, respectively. They are mounted in the portable phone.
As document information of the conventional art related to the present invention, U.S. Pat. No. 5,926,139 and “New antenna engineering”, by Hiroyuki Arai, Sogo Electronics Press, Apr. 9, 1996, p109–p 114 are known, for example.
In the conventional antenna device, however, radiant efficiency of each antenna element must be increased in order to prevent the band width having desired sensitivity in a frequency band used in the portable phone, namely a so called specific band, from becoming narrow. The radiant efficiencies can be increased when shape sizes of first antenna element 21 and second antenna element 22 are increased to enlarge projection area. However, this method goes against the recent demand for downsizing a radio device. In this conventional configuration, however, it is difficult to provide a built-in type multi-band-compatible antenna device that is downsized and has high radiant efficiency.
The present invention addresses such a conventional problem, and provides a built-in type antenna device that can easily support many bands, can be downsized, and has antenna elements having balanced radiant efficiency.